432 
Marine Cercariae 
Cercaria ubiquita is common in Paludestrina stagnalis at Fenham Flats 
and Loch Ryan on the West Coast of Scotland. Occasionally also I 
have found it in Littorina obtusata and Littorina rudis. At Millport 
Littorina obtusata from the rock pools wliere the crabs live is its usual 
host. It occupies the digestive gland in oval sporocysts and the whole 
of the organ has an unhealthy yellowish colour and can generally be 
quite easily told from an ordinary healthy digestive gland. 
The sporocysts (fig. 3) are oval and of various sizes, the usual size 
being about 0'6 mm. long. They are pinkish yellow when together but 
almost colourless when looked at singly. Cercariae in all stages are 
contained within which easily burst out of the thin-walled sporocyst. 
The cercaria (fig. 4) measures 0T2 mm. in length without its tail 
which is about the same length as the body and very thin and active. 
It is extremely transparent. The oral sucker measures about O’OSO mm. 
across but often appears oval when extended. It is armed with a long 
pointed stylet and by its sides open the four ducts (two each side) of the 
stylet glands which run up the body springing from two masses of large 
cells which occupy the greater part of the body. No ventral sucker is 
to be seen. It apparently develops afterwards as does also the 
alimentary canal. The excretory vesicle is bilobed. The tail very 
easily detaches itself and moves about separately. The cercaria is fond 
of doubling itself up with its fore and hind ends doubled in so that it 
looks like a neatly folded parcel. It apparently leaves its first host and 
swims freely about in the water, then bores its way by means of its 
stylet and glands into the tissues of the crab. The edible crab Cancer 
pagurus is also an intermediate host for this worm. I have found 
cercariae exactly corresponding with G. ubiquita,, but without the glands, 
inside the tissues of Garcinus maenas. I have also found it curled up 
and surrounded by a very thin cyst, 0‘08 mm. across (fig. 5). It has 
now lost its tail but the stylet remains for some time longer. It 
occupies almost every tissue of the crab, liver, muscles, gonad and 
outside the blood vessels. Having settled down it grows considerably 
and the cyst with it, but the latter however is still very thin-walled. 
The stylet is lost when the cyst measures about 0’30 mm. across. The 
ventral sucker and alimentary canal appear and the body spines begin 
to form. The worm stops growing when the cyst is about 0'35 mm. 
a'cross and tjien the cyst wall becomes very thick, 0'02 mm. thick and 
the real resting stage begins. The cercaria is now of the ordinary 
Spelotrema form. The usual size of the thick-walled cyst is 0'4-0‘48 
mm. across. It is very difficult to get the enclosed worm out of the 
